We will open the meeting by reviewing what is currently known about
the star formation and chemical evolution histories of the dwarf
galaxies in the Local Group. For if we are to claim to understand the
physics of galaxy evolution, we must first be able to successfully
model these, the simplest of galaxies. Because of their close
proximity, we can determine ages and chemical compositions on a
star-by-star basis and accurately determine their evolution from t=0
to the present day (with varying degrees of time resolution). The
density of stars in regions of the color-magnitude diagram (CMD),
particularly at main-sequence turnoff, gives us the star formation
history. We can hope to disentangle the degeneracy of age-metallicity
in these galaxies (as opposed to more distant ones for which only
integrated light is available) by determining metallicity from the
color of the giant branch, or by explicitly determining the
metallicity distribution from spectra of individual stars. Using these
methods, we are finding that, contrary to past prejudices, even the
dwarf spheroidals in the Local Group have had complex evolutionary
histories.

The evolution of the Carina, Fornax, Leo I and Leo II dwarf spheroidals
and the Large Magellanic Cloud, which span a wide range in galaxy luminosity
from M_v=-9 to -18, will be discussed in detail. (I have chosen these because
CMDs reaching the critical main-sequence turnoff region have been
obtained for each.) In these galaxies, episodic "bursts" of star formation
appear to be common. I use quotes around bursts to highlight the fact that
we do not yet know how bursty these were. Theoreticians have argued that
low luminosity dwarfs form at early times on a cloud cooling timescale,
typically 10^7 yrs. However, we will argue that evidence points to these
bursts happening on timescales of greater than, or of order, 10^8 yrs with
duty cycles of ~5 Gyr. The duration of the bursts and the duty cycle have
obvious implications for the contribution of intrinsically low luminosity
dwarfs to the faint blue galaxy counts.

My review talk will consist of:
1) Carina dSph (M_V=-9)
The Carina dSph has had bursts of star formation 3, 5, 6 to 8 and
~15 Gyr ago with little or no star formation between these episodes.
Roughly 70% of the stars are of intermediate-age and 15% are
~15 Gyr. The colors of stars on the giant branch imply the mean
metallicity is [Fe/H]=-1.8 and the dispersion is low (~< 0.2 dex).
The lack of chemical enrichment and the high M/L ratio (32 Mo/Lo),
suggests that galactic winds ejected gas through out the evolution
of the galaxy and lead to an inefficient conversion of gas to stars.
references: Mighell (1990) A&AS
Mighell & Butcher (1992) A&A
Smecker-Hane, et al. (1994, 1996) - AJ and in progress
Mateo et al (1993) AJ
Carina illustrates some puzzling problems that are ubiquitous to
dwarf galaxies:
- The energy input from the ~10^3 supernovae that
exploded during the first episode of SF is much
larger than the binding energy of the galaxy (even with
90% of the mass in a dark matter halo) -- naively, this
galaxy should have ejected its ISM during the first SF
episode.
- But it did not. Multiple episodes of SF occurred
so either ejection of gas was inefficient, or new
gas was accreted. The observed metallicity argue for
pristine gas, rather than gas that had been ejected
during the first burst of SF. However, accretion
would be difficult because the total mass is very low
(10^7 Mo), the cross section is small (r_t=660 pc) and
the kinematic speed in the outer halo is ~220 km/s,
hence for gas to successfully merge with the galaxy
it would have had to be moving in nearly the same orbit.
I will briefly discuss my objections to the idea
put forth by Lin and Murray (1994) that Carina is the
remains of a piece ripped off a larger-LMC galaxy, in which
a second episode of SF was triggered by subsequent
accretion of gaseous tidal debris.
- I will argue that the best explanation for such an episodic
SF rate is that cooling and SF operates slowly (on ~ 10^8 yr)
and ejection of gas is inefficient. Supernovae drive winds
which rapidly accelerate the low density
ISM. However, because of the small physical
size and diffuseness of the galaxy, the wind rapidly
breaks through holes in the ISM -- leaking out metals, heat,
energy and momentum -- but only slowly disrupt the densest
molecular gas clouds. I argue that SF in the first "burst"
occurs slowly over ~10^8 yr, some gas remains in the galaxy
after SF is quenched, and the gas stays below the threshhold
for SF for a few Gyr. Exactly what causes the few Gyr
timescale between bursts remains to be answered -- maybe
the remaining gas is optically thin to the UV background
and remains photoionized for long times, or maybe
tidal torques or increased ambient pressure can repeatedly
nudge it over the threshold for SF (the suspected orbital
peroid around the Galaxy is ~ few Gyr).
2) Leo I dSph (M_V=-10)
A CMD of Leo I obtained with HST compliments the ground-based CMD and
shows that Leo I also has experienced two distinct epochs of SF.
Roughly 90% of the stars have an age of ~3 Gyr, and ~10% have ages
of ~> 12 Gyr.
references: Lee et al (1993), AJ
Mateo et al (1994) BAAS
3) Leo II dSph (M_V=-12)
The CMD of Leo II obtained with HST shows a period of extended SF
from 7 to 14 Gyr ago. However, because of its larger distance,
subsequently larger photometric errors, and the narrowing of the
isochrones for older ages, it is unclear whether how constant
the SFR was during this interval.
reference: Mighell et al (1996), ApJ
4) Fornax dSph (M_V=-14)
Fornax dSph has a higher luminosity, mass, surface density and a
retinue of 5 globular clusters. The latest CMDs of Fornax show that it
has had an extended SF history. Stars began to form early, ~15 Gyr ago.
There are hints that the SFR rose between 5 and 8 Gyr ago, and a
trickle of stars continued to form until very recently. Indeed, there
is a small population of very young (few 10^8 yr old) stars. Unlike
lower mass dSphs, Fornax stars show a wide range of metal abundances
from -1.7 < [Fe/H] < -0.7. Its lower M/L (~10 Mo/Lo) suggests a
more efficient conversion of gas to stars.
references: Buonanno et al (1985), A&A
Beauchamp et al (1995), AJ
Smecker-Hane et al (1996), in preparation
Stetson et al (1996), in preparation
Mateo et al (1991), AJ
5) Large Magellanic Cloud (M_V=-18)
For many years, the uneven distribution of LMC star cluster ages
(0.1 Gyr, ~3 to 4 Gyr, and ~12 to 15 Gyr) hinted at a non-constant
SFR. Ground-based CMDs of LMC field stars have been analyzed.
Because of the limitations of the photometry (driven by crowding),
an unknown mix of metallicities, and possible inadequacies of
stellar evoln models, the Padova group have advocated "region-fitting"
to quantify the LMC's SFR. They count stars in key regions of the
CMD that are age sensitive to test simple models of the SFR. Assuming
a constant SFR + one burst, they determine that the best fit model
is one in which a burst that increased the SFR by a factor of 10
occurred ~ 2 to 4 Gyr ago (with mean age possibly varying with position
across the LMC) and having a duration of ~10^9 yr or more. Although
the data hint that this simple model (a single burst) may be
oversimplified.
In support of these results, a new CMD of field stars obtained with
HST shows structure in the main-sequence turnoff region - evidence of
an episodic SFR. A burst with an age of ~2 Gyr, duration 10^8 yr,
superimposed on a nearly constant SFR in the last 1 to 3 Gyr is derived.
Of order 25% of the stars may have formed in the 2 Gyr burst with 80%
of the stars forming from 1 to 3 Gyr ago. This was preceded by low SFR
in the preceeding few Gyr. The mean age of the older disk population is
~8 Gyr and with a very low fraction of disk stars being ~15 Gyr old.
Complimenting this, new results on the age and chemical abundances of
planetary nebula in the LMC show that a rapid increase in the chemical
abundances (factor of 2) occurred 2 Gyr ago.
But these data/analysis do not preclude an more complex evolution at
ages ~> 4 Gyr. In fact, they hint that the single burst picture is not
complete. It is imperative that we use HST to overcome crowing
problems at faint mags, in combination with ground based photometry
and spectroscopic metallicity determinations of bright evolved stars,
if we are to resolve the evolutionary history of one of our nearest,
most luminous, neighbors.
references: Bertelli et al (1992), ApJ
Westerlund, Linde & Lynga (1995), A&A
Vallenari et al (1996a, 1996b), A&As, in press
Gallagher et al (1996), ApJ, in press
Smecker-Hane et al (1996), in progress
Dopita et al (1996), in progress